The invention described herein relates to nuclear reactor fuel assemblies and more particularly to an improved design of top and bottom nozzles which increase fuel assembly performance.
Conventional commercial nuclear reactors include a multiplicity of fuel assemblies, each including a parallel array of fuel rods and control rod guide thimbles held in radially spaced relationship with each other by grids spaced along the fuel assembly length. To provide strength and stability to the assembly, the guide thimbles are secured to the grids and to top and bottom nozzles mounted on opposite ends of the assembly. Each nozzle is of solid one piece construction equipped with openings to accept the guide thimble ends and to accommodate coolant flow upwardly through the assembly during reactor operation. The ligaments utilized to form the nozzle coolant openings are so located above fuel rods to limit their upward movement or even their escape from the assembly when acted on by hydraulic forces in the reactor.
Current fuel assembly end nozzles are designed to carry the structural and thermal loads during both handling and operation and to control the flow of coolant through the ends of the fuel assembly. Also, the nozzles provide the structure to which the control rod guide thimbles are secured to furnish stability to the assembly while returning the rods within the confines of the assembly. The requirements are satisfied by utilizing nozzles of a single or one piece construction which includes spaced plates joined by side walls to form the nozzle body. This one piece design of nozzle used the same features in an attempt to satisfy all the above requirements. As a result, the design features incorporated in the nozzle became complex and expensive to manufacture particularly since different structural features could not be changed at the expense of others, i.e., the proposed design changes in each structural component could not be made if they adversely affect the function or performance of other components in the nozzle. For example, the ligaments which act to preclude upward displacement of fuel rods from the assembly also constitute thermal and hydraulic load bearing members, thus requiring the ligaments to be made substantially heavier than would otherwise be necessary. The nozzle manufacturing process also is unusually expensive since the multitude of flow openings must be cut milled to precise configurations and tolerances, which requires machines having large cutting radiuses plus still being capable of achieving fine dimensions or tolerances.